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Extract black and white image from android camera's NV21 format

Posted by: admin April 23, 2020 Leave a comment

Questions:

I have done some google-ing around and couldn’t find enough information about this format. It is the default format for camera preview. Can anyone suggest good sources of information about it and how to extract data from a photo/preview image with that format? To be more specific, I need the black and white image extracted.

EDIT: Seems like that format is also called YCbCr 420 Semi Planar

How to&Answers:

I developed the following code to convert the NV21 to RGB, and it is working.

/**
 * Converts YUV420 NV21 to RGB8888
 * 
 * @param data byte array on YUV420 NV21 format.
 * @param width pixels width
 * @param height pixels height
 * @return a RGB8888 pixels int array. Where each int is a pixels ARGB. 
 */
public static int[] convertYUV420_NV21toRGB8888(byte [] data, int width, int height) {
    int size = width*height;
    int offset = size;
    int[] pixels = new int[size];
    int u, v, y1, y2, y3, y4;

    // i percorre os Y and the final pixels
    // k percorre os pixles U e V
    for(int i=0, k=0; i < size; i+=2, k+=2) {
        y1 = data[i  ]&0xff;
        y2 = data[i+1]&0xff;
        y3 = data[width+i  ]&0xff;
        y4 = data[width+i+1]&0xff;

        u = data[offset+k  ]&0xff;
        v = data[offset+k+1]&0xff;
        u = u-128;
        v = v-128;

        pixels[i  ] = convertYUVtoRGB(y1, u, v);
        pixels[i+1] = convertYUVtoRGB(y2, u, v);
        pixels[width+i  ] = convertYUVtoRGB(y3, u, v);
        pixels[width+i+1] = convertYUVtoRGB(y4, u, v);

        if (i!=0 && (i+2)%width==0)
            i+=width;
    }

    return pixels;
}

private static int convertYUVtoRGB(int y, int u, int v) {
    int r,g,b;

    r = y + (int)(1.402f*v);
    g = y - (int)(0.344f*u +0.714f*v);
    b = y + (int)(1.772f*u);
    r = r>255? 255 : r<0 ? 0 : r;
    g = g>255? 255 : g<0 ? 0 : g;
    b = b>255? 255 : b<0 ? 0 : b;
    return 0xff000000 | (b<<16) | (g<<8) | r;
}

This image helps to understand.
YUV420 NV21

If you wanna just grayscale image is easer. You can discard all the U and V info, and take just the Y info. The code would can be like this:

/**
 * Converts YUV420 NV21 to Y888 (RGB8888). The grayscale image still holds 3 bytes on the pixel.
 * 
 * @param pixels output array with the converted array o grayscale pixels
 * @param data byte array on YUV420 NV21 format.
 * @param width pixels width
 * @param height pixels height
 */
public static void applyGrayScale(int [] pixels, byte [] data, int width, int height) {
    int p;
    int size = width*height;
    for(int i = 0; i < size; i++) {
        p = data[i] & 0xFF;
        pixels[i] = 0xff000000 | p<<16 | p<<8 | p;
    }
}

To create your Bitmap just:

Bitmap bm = Bitmap.createBitmap(pixels, width, height, Bitmap.Config.ARGB_8888);

Where pixels is your int [] array.

Answer:

NV21 is basically YUV420 but instead of planar format where Y, U and V have independent planes, NV21 has 1 plane for Luma and 2nd plane for Chroma. The format looks like

YYYYYYYYYYYYYYYYYYYYYYYYYYYYY
YYYYYYYYYYYYYYYYYYYYYYYYYYYYY
.
.
.
.

VUVUVUVUVUVUVUVUVUVUVUVUVUVUVU
VUVUVUVUVUVUVUVUVUVUVUVUVUVUVU
.
.
.
.
.

Answer:

I also had lots of headache because of this preview format.

The best I could find are these:

http://www.fourcc.org/yuv.php#NV21

http://v4l2spec.bytesex.org/spec/r5470.htm

It seems that the Y component is the first width*height bytes int the array you get.

Some more informational links:

http://msdn.microsoft.com/en-us/library/ms867704.aspx#yuvformats_yuvsampling

http://msdn.microsoft.com/en-us/library/ff538197(v=vs.85).aspx

Hope this helps.

Answer:

The data is in YUV420 format.
If you are only interested in the monochrome channel, i.e. “black and white”, then this the first width x height bytes of the data buffer you already have.
The Y channel is the first image plane. It is exactly the grey/intensity/luminosity etc. channel.

Answer:

Here’s code to just extract the greyscale image data:

private int[] decodeGreyscale(byte[] nv21, int width, int height) {
    int pixelCount = width * height;
    int[] out = new int[pixelCount];
    for (int i = 0; i < pixelCount; ++i) {
        int luminance = nv21[i] & 0xFF;
        out[i] = Color.argb(0xFF, luminance, luminance, luminance);
    }
    return out;
}

Answer:

When you only need a grayscale camera preview, you could use a very simple
renderscript:

# pragma version(1)
# pragma rs java_package_name(com.example.name)
# pragma rs_fp_relaxed

rs_allocation gIn;   // Allocation filled with camera preview data (byte[])
int previewwidth;    // camera preview width (int)

// the parallel executed kernel
void root(uchar4 *v_out, uint32_t x,uint32_t y){
   uchar c = rsGetElementAt_uchar(gIn,x+y*previewwidth); 
   *v_out = (uchar4){c,c,c,255};
}

Note : This is not faster than ScriptIntrinsicYuvToRGB (and a following ScriptIntrinsicColorMatrix to do the RGBA-> gray), but
it runs with API 11+ (where the Intrinsics need Api 17+).